It is known that structures of this type are used frequently in articles such as tires, conveyor belts, transmission belts, flexible hose, etc.
In such articles, the metal wires, possibly in the form of cords, must, at one and the same time, have adequate mechanical features, good resistance to corrosion and a good adhesion to the elastomeric material.
Steel, the material of choice with respect to mechanical features, has the disadvantage of adhering insufficiently to the vulcanized elastomeric material. In order to obtain a high level of adhesion to the vulcanized elastomeric material it is thus necessary to coat the steel with suitable materials.
For instance, it is known that it is possible to obtain good adhesion with steel wires coated with a layer of brass made up of about 70% of copper and 30% of zinc. In this case the connection is promoted by the formation of a thin layer of copper sulphide, by the sulfur itself or by its known derivatives present, as vulcanizing agents, in the vulcanization mixture.
Adhesion failure between brass coated steel and elastomer occurs in the presence of humidity and oxygen. Decay can be reduced in part by adding some cobalt in the form of a salt. In this case it is believed that cobalt reduces the electrical conductivity of the layer of copper sulphide, slowing the rate of diffusion of the Zn++ ions and, as a consequence, the interface layer ZnO/Zn(OH).sub.2 responsible for the destruction of the Cu.sub.x S film is reduced. As a result, improvement in adhesion is obtained.
While the condition of ensuring a connection that is good and stable in time is, on the whole, satisfied, brass-coated steel is only slightly resistant to corrosion.
In the articles of vulcanized elastomeric material reinforced with brass-coated steel wires, possibly in the form of cords, such as tires, corrosion is induced both by humidity which permeates the elastomer and by water which manages to come into contact with the brass-coated steel reinforcing structure through holes or tears in the elastomer.
In the case of articles comprising metal wires embedded in a vulcanized elastomeric matrix, there still exists a great demand to satisfy two distinct requirements: (1) to ensure a good wire/vulcanized elastomeric matrix connection and (2) good corrosion resistance of the wire.
An apparently simple solution would be to coat the steel wires with a metal, such as, for example, zinc, which is capable of preventing corrosion. However, experiments carried out by the Applicants with mixtures which provide a good adhesion, as in the case of brass-coated steel cords, have proven to exhibit unsatisfactory results on galvanized steel cords.
The Applicants have therefore carried out other experiments raising the concentrations of the components capable of promoting good adhesion between the elastomer with brass-coated cords. More particularly the following have been used:
a high concentration of cobalt (about 0.5-1%, as metallic cobalt) by weight with respect to the weight of elastomer in the mixture;
high concentrations of resins that form in the mixture during vulcanization by resorcinol reaction in the presence of donors of methylene groups (resorcinol concentration in the mixture before vulcanization=about 4-10% by weight with respect to the weight of elastomer in the mixture); and/or
a high concentration of sulphur (at least about 6% by weight with respect to the weight of elastomer in the mixture);
In all cases articles have been obtained whose features are on the whole inferior to those of the brass-coated cord/conventional mixture articles.
In fact, the mixtures with high cobalt concentrations provide connections with limited resistance to overvulcanization and to aging at high temperatures; mixtures with high resin concentrations pose environmental problems (generation of fumes) and connections with slight resistance to aging; mixtures with high sulfur concentrations make processing difficult and have limited resistance to aging.
The Applicants have therefore experimented with the behavior of products known for their capacity to facilitate adhesion between elastomers and metal surfaces such as, for example, the mercaptotriazines.
But even these substances, on the other hand, have not given satisfactory results with conventional galvanized steel/elastomeric matrices cords or for brass-coated steel cords.
But it has unexpectedly been found that there is a considerable synergy of action between cobalt and trimercaptotriazine such as to promote the maintenance of good connection levels even after aging and in the presence of humidity between elastomers and galvanized steel wires.